Re: [PATCH] Revert "mm:vmscan: fix inaccurate reclaim during proactive reclaim"

[Michal Hocko <mhocko@suse.com>]

On Tue 23-01-24 11:48:19, Johannes Weiner wrote:
> The revert isn't a straight-forward solution.
> 
> The patch you're reverting fixed conventional reclaim and broke
> MGLRU. Your revert fixes MGLRU and breaks conventional reclaim.
> 
> On Tue, Jan 23, 2024 at 05:58:05AM -0800, T.J. Mercier wrote:
> > They both are able to make progress. The main difference is that a
> > single iteration of try_to_free_mem_cgroup_pages with MGLRU ends soon
> > after it reclaims nr_to_reclaim, and before it touches all memcgs. So
> > a single iteration really will reclaim only about SWAP_CLUSTER_MAX-ish
> > pages with MGLRU. WIthout MGLRU the memcg walk is not aborted
> > immediately after nr_to_reclaim is reached, so a single call to
> > try_to_free_mem_cgroup_pages can actually reclaim thousands of pages
> > even when sc->nr_to_reclaim is 32. (I.E. MGLRU overreclaims less.)
> > https://lore.kernel.org/lkml/20221201223923.873696-1-yuzhao@google.com/
> 
> Is that a feature or a bug?
> 
>  * 1. Memcg LRU only applies to global reclaim, and the round-robin incrementing
>  *    of their max_seq counters ensures the eventual fairness to all eligible
>  *    memcgs. For memcg reclaim, it still relies on mem_cgroup_iter().
> 
> If it bails out exactly after nr_to_reclaim, it'll overreclaim
> less. But with steady reclaim in a complex subtree, it will always hit
> the first cgroup returned by mem_cgroup_iter() and then bail. This
> seems like a fairness issue.

Agreed. We would need to re-introduce something like we used to have
before 1ba6fc9af35bf.

> We should figure out what the right method for balancing fairness with
> overreclaim is, regardless of reclaim implementation. Because having
> two different approaches and reverting dependent things back and forth
> doesn't make sense.

Absolutely agreed!

> Using an LRU to rotate through memcgs over multiple reclaim cycles
> seems like a good idea. Why is this specific to MGLRU? Shouldn't this
> be a generic piece of memcg infrastructure?
> 
> Then there is the question of why there is an LRU for global reclaim,
> but not for subtree reclaim. Reclaiming a container with multiple
> subtrees would benefit from the fairness provided by a container-level
> LRU order just as much; having fairness for root but not for subtrees
> would produce different reclaim and pressure behavior, and can cause
> regressions when moving a service from bare-metal into a container.
> 
> Figuring out these differences and converging on a method for cgroup
> fairness would be the better way of fixing this. Because of the
> regression risk to the default reclaim implementation, I'm inclined to
> NAK this revert.

I do agree that a simple revert doesn't seem to be the way to go.

-- 
Michal Hocko
SUSE Labs